I'm seeing the same problem for all cxComboBoxes with property DropDownListStyle=IsFixedList

It seems TsiLang sets the Text property of the cxComboBox to the new language. Then the cxComboBox searches this value within its Items, doesn't find it and as a result sets ItemIndex to -1 because IsFixedList doesn't allow values not contained in the Items list.

If this is the case then as a solution could be just to exclude cxComboBox.Text property from translation (as it could be included automatically) and it will be translated by translating Items property content.